The Android 2.3.4 Update for Nexus S which was recently released would make video and voice chat feasible over 3G/4G or Wi-Fi. This was the official statement released by Google that would bring Google Talk for some of the Android smart phones.
Sadly, in other words if you possess an Android with a front facing camera then you will not be able to avail this facility just yet and may have to have patience for a few months longer.
Google has officially established video and voice chat functionality with Android 2.3.4 update. Though sadly as of now only Nexus S can boast of this update over-the-air. Once Android 2.3.4 update is established on the phone, you will get to view videos or voice chat button in your Gtalk friend list. It may have some restrictions at the outset and you may find that this does not work as efficiently on devices with a reduced amount of RAM.